In the current commercial process used for making polyethylene film-fibril sheets (e.g., Tyvek.RTM. spunbonded polyolefin sheets commercially available from E. I. du Pont de Nemours and Co. of Wilmington, Del.), continuous fibers having a desired strength, fineness and surface area are produced by flash spinning a solution of high density polyethylene (HDPE) in a trichlorofluoromethane ("Freon 11" or "F-11") spin agent. The importance of using a spinneret and tunnel configuration on imparting key properties, such as tenacity and elongation to break, to the flash spun, continuous fibers is described in U.S. Pat. No. 3,081,519 (Blades et al.), U.S. Pat. No. 3,227,794 (Anderson et al.) and U.S. Pat. No. 4,352,650 (Marshall). In particular, Marshall discusses the optimization of tunnel configuration for increasing the fiber tenacity (e.g., from 4.2 to 5.2 grams per denier) of flash spun, continuous fibers, while eliminating certain defects caused by high throughput conditions under non-optimum tunnel configurations. In general, fiber tenacity can be increased by as much as 1.3 to 1.7 times by using a tunnel at the spinneret exit. However, although these prior art methods work well for making continuous fibers, there is no mention of how to make strong discrete (i.e., discontinuous) fibers using flash spinning equipment.
In the past, various methods have been suggested for making discrete fibers using a secondary fluid. However, none of these methods produce discrete fibers having acceptable strength for such things as paper and cement reinforcment applications. The major use of these prior art discrete fibers has been as a fusing component in cellulosic pulp. Due to this use, no effort has been made to orient the polymer matrix during flash spinning and to fragment the matrix at the appropriate moment. Examples of these prior art methods include U.S. Pat. Nos. 4,025,593; 4,600,545; 4,189,455; and 4,642,262.
Clearly, what is needed is a method for making strong discrete fibers that does not have the problems and deficiencies inherent in the prior art. In particular, it is an object of the present invention to produce strong discrete fibers of a desired quality (e.g., strength, average length, fineness and surface area) using standard flash spinning equipment. Other objects and advantages of the invention will become apparent to those skilled in the art upon reference to the drawings and the detailed description of the invention which hereinafter follow.